This invention relates to an alloy steel characterized by an outstanding combination of strength and hardness and, more particularly, to such an alloy which is readily balanced to provide a unique combination of toughness, ductility and hardness.
Alloy steels have hitherto been provided which have had good toughness and ductility combined with high strength, but such alloys have left much to be desired. For example, in an effort to maximize secondary hardness, that is the hardening effect provided by the precipitation of fine carbides from the martensitic matrix during tempering, the parts fabricator is lead to use high austenitizing temperatures. While this may provide a higher degree of hardness, it also usually results in unacceptably coarse grain structures in the heat-treated part. The increasingly more general use of vacuum heat-treating furnaces is believed to have resulted in more frequent occurrence of this problem of excessive grain coarseness. This may be best illustrated by considering a well known alloy steel type A.I.S.I. M50 containing 0.80% carbon, 0.25% manganese, 0.25% silicon, 4.00% chromium, 1.00% vanadium, 4.50% molybdenum and the balance iron except for incidental impurities, used in the manufacture of bearings. If in order to maximize heat-treated hardness and consistently attain a minimum room temperature hardness of R.sub.c 60 and a minimum hot hardness of R.sub.c 45 at 1000.degree. F. to enhance bearing life, bearing manufacturers exceed the permissible austenitizing temperature range of 2000.degree. to 2050.degree. F., an overheated coarse microstructure results which is brittle.
A similar problem has been encountered in connection with the fabrication of band saw blades from M50 alloy steel where at least the teeth forming portion of the blade must have high hardness and wear resistance. While a room temperature hardness of about R.sub.c 60-61 was attainable, it suffered from poor blade life believed to be caused by the presence of excessively large grains.